In the tandem-type image forming apparatuses such as copiers or printers that form a color image, image forming devices of respective colors are disposed side by side to form images of respective colors on an image carrier. The images of respective colors are transferred from the image carrier onto a recording sheet one above the other to form a color composite image. In operation, the tandem-type image forming apparatuses transfer image data of respective colors through a network or an interface between devices, or write in or read out image data of respective colors to or from a memory. In order to increase efficiency in image data transfer or image data storage, the image data of respective colors may be each encoded.
In such case, the image forming apparatuses need to decode the image data by the time when a recording sheet is transferred to an image transfer section. If the decoded image data is not available by then, the image forming apparatuses may need to control a sheet transfer device to transfer the recording sheet at lower speeds or hold operation of transferring the recording sheet, for example, as described in Japanese Patent Application Patent No. 3721612. This, however, increases the overall processing time required for printing, especially when a large number of images are to be successively formed.